Many electronic circuits incorporate voltage reference circuits. Bandgap reference generator circuits are widely utilized to generate a bandgap reference voltage that has a negligible temperature coefficient and is independent of temperature (i.e., that should remain constant and stable regardless of changes in temperature). Thus, it's highly desirable that the bandgap reference voltage is substantially independent of temperature variations, or stated differently that a low temperature coefficient (TC).
FIG. 1 is a circuit schematic that shows a conventional bandgap reference generator circuit 100. The bandgap reference generator circuit 100 is connected to a supply voltage (VDD) 105 at node A and generates a bandgap reference voltage (VBG) 125 at node E. The bandgap reference generator circuit 100 includes a P-channel metal oxide semiconductor field effect transistor (MOSFET) 110, a first resistor (R1) 130 having a first resistance value, a second resistor (R2) 140 having a second resistance value, a third resistor (R2) 150 having the second resistance value, an operational amplifier 170, a first bipolar junction transistor 180, and a second bipolar junction transistor 190.
The P-channel MOSFET 110 includes a source terminal coupled to a supply voltage (VDD) 105 at node A, a control terminal or gate coupled to an output of the operational amplifier 170 at node C and a drain terminal coupled to node E.
The operational amplifier 170 includes an inverting input, a non-inverting input, and an output. The operational amplifier 170 receives a voltage generated at node G at its inverting input and another voltage generated at node H at its non-inverting input, and based on these inputs generates an output voltage (Vout) at its output. The output voltage generated by the operational amplifier 170 is applied at the gate terminal of MOSFET 110. When the MOSFET 110 is operating in its saturation region, the MOSFET 110 operates as a current source and generates a current (I) that is output from its drain terminal to node E.
The bandgap reference generator circuit 100 includes a first branch 122 and a second branch 124. The first branch 122 includes the first resistor 130 that is coupled to a first PNP bipolar junction transistor (BJT) 180 at node H. The second branch includes the second resistor 140 that is coupled to the third resistor 150, and the third resistor 150 is coupled to the emitter terminal of a second PNP bipolar junction transistor (BJT) 190. The base and collector terminals of the first and second bipolar junction transistors 180, 190 are coupled to ground 195. The PN junction area (or size) of the first bipolar junction transistor 180 is N times smaller than the PN junction area of the second bipolar junction transistor 190. In one exemplary implementation, the integer N is equal to eight, which means that the bipolar junction transistor 190 is equivalent to eight instances of the first bipolar junction transistor 180. As such, in this example, the ratio of the PN junction area of the second bipolar junction transistor 190 and the PN junction area of the first bipolar junction transistor 180 is 8:1.
The current (I) generated at the drain terminal of MOSFET 110 flows into node E and splits into current (I1) that flows through the first branch 122 and a current (I2) that flows through the second branch 124. The portion (I2) of the current (I) that flows through the second branch 124 generates the bandgap reference voltage 125. The bandgap reference voltage 125 can be approximated as shown in expression (1) as follows:VBG≈VBE+17.2×VT  (1).
Ideally, it is desirable that the temperature coefficient (TCVBG) of the bandgap reference voltage 125 is as close to zero as possible. The temperature coefficient (TCVBG) of the bandgap reference voltage 125 can be represented in expression (2) as follows:TCVBG=TCVBE1+17.2×TCVT=0  (2),
where the temperature coefficient (TCVBE1) of the base-to-emitter voltage (VBE) of the first PNP bipolar junction transistor (BJT) 180 is approximately −1.5 mV/° K and where the temperature coefficient (TCVT) of the thermal voltage (VT) is approximately 0.087 mV/° K.
The difference between the first base-to-emitter voltage (VBE1) and the second base-to-emitter voltage (VBE2) can be expressed in expression (3) as follows:ΔVBE=VBE1−VBE2=VT×ln N  (3).
Further, the current (I2) 214 that flows along branch 124 can be represented in expression (4) as follows:
                              I          2                =                                                            V                                  BE                  ⁢                                                                          ⁢                  1                                            -                              V                                  BE                  ⁢                                                                          ⁢                  2                                                                    R              ⁢                                                          ⁢              3                                =                                                    Δ                ⁢                                                                  ⁢                                  V                  BE                                                            R                ⁢                                                                  ⁢                3                                      .                                              (        4        )            
In FIG. 1, the bandgap reference voltage (VBG) 125 can be approximated via expressions (5) and (6) as follows:
                                          V            BG                    ≈                                    V                              BE                ⁢                                                                  ⁢                2                                      +                                          I                2                            ×                              (                                                      R                    3                                    +                                      R                    2                                                  )                                                    ,                            (        5        )                                          V          BG                ≈                              V                          BE              ⁢                                                          ⁢              2                                +                                                                      V                  T                                ×                ln                ⁢                                                                  ⁢                N                                            R                3                                      ⁢                                          (                                                      R                    3                                    +                                      R                    2                                                  )                            .                                                          (        6        )            
The bandgap reference generator circuit 100 works well in many applications, but does not operate as expected in other applications. For the bandgap reference generator circuit 100 to work properly, the MOSFET 110 must operate in its saturation region as a current source. However, when a supply voltage (VDD) 105 is too low, the MOSFET 110 will operate its linear region and the bandgap reference generator circuit 110 will not produce the bandgap reference voltage 125 that is required. For example, when the supply voltage (VDD) 105 that is utilized in the circuit becomes a low value, for example, 1.35 V, the MOSFET 110 operates its linear region and no longer operates the current source. Because the MOSFET 110 cannot operate in its saturation region with this low supply voltage (VDD) 105, the resulting bandgap reference voltage (VBG) 125 is no longer high enough and cannot satisfy the relationship of expression (6) (above).
For instance, in one implementation, where the base to emitter voltage (VBE2) of the second bipolar junction transistor 190 is 0.75 V, the thermal voltage is 0.029 V, N is equal to eight (8), the first resistance value of the first resistor (R1) 130 is 72 kΩ and the second resistance value of the second resistor (R2) 140 and the third resistor (R2) 150 are 10 kΩ, then the bandgap reference voltage (VBG) 125 will only be 1.25 V.
Accordingly, it is desirable to provide improved bandgap reference generator circuits that are capable of working with lower power supply voltages (VDD) (e.g., 1.5 volts or less). It would also be desirable if such a bandgap reference generator circuit is capable of generating a lower bandgap reference voltage (e.g., 0.8 volts or less) having a low temperature coefficient (e.g., near zero, for example, 12 parts per million or less). It would also be desirable if such a bandgap reference generator circuit can be implemented using MOSFET technology that consumes less current. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.